Hello, This behavior has been documented on my Apple post:
http://discussions.apple.com/thread....readID=1106671
Log into your AirportExtreme (AE) unit > Go to the disk section > Click the "Disconnect All Users" Button > Then Restart the AE with the restart option from the menu, don't power cycle it. That should restore use of your air port disk.
There are several actions that can cause this behavior. One of them is a small power outage. you should put the AE unit and the USB drive on a UPS. Read through the Apple post and post that your also having issues and Call Apple and file a support case with them.